With regards to his 3rd Meditation and 5th Meditation proofs of God, I can't seem to find any reason to see one as being more successful than the other.

Mainly because while his ontological proof rests on his assumption of our having clear and distinct perceptions, this in turn depends on his first argument of God’s existence, which depends on us sharing an innate idea of a perfect infinite God.

The two arguments seem to interact such that clear and distinct perceptions are made certain because God exists, and God's existence, as an essential property, is clearly and distinctly perceived. However, this only reinforces the Cartesian circle.

So the premises of his fifth meditation argument depend upon assumptions he previously makes in his trademark argument (that innate knowledge of God implies His existence) so both arguments seem to be equally refutable when we bring doubt to the premises used to justify the doctrine of innate ideas and of clear and distinct perceptions.

I would be grateful if anyone could point out to me any strengths/weaknesses of either argument that would help me in some way to decide which proof is more compelling (even if they are both poor!).

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    Why "successful" ? They have been widely debated since Descartes time. See e.g. Descartes' Ontological Argument. Apr 14 '18 at 11:48
  • I suppose I am questioning whether one argument should be considered more sound than another- to me, both are based on similarly shaky premises (thanks to their reliance on his doctrine of clear and distinct perceptions and innate ideas) are so are equally unsound. Apr 14 '18 at 11:59
  • Up to now, NO argument about the existence of God can be "ultimative" i.e. considered 100% logically valid and sound (i.e. based on true premises). Apr 14 '18 at 12:01
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    I do not believe Descartes made any decent argument for God. He seems to be largely taken for granted.
    – user20253
    Apr 14 '18 at 15:15
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    Can one argument be 'more' successful than another ? An argument is successful or it is not. How do degrees of success enter the picture ? Agreed, one inductive argument can be more probable that another. But Descartes' arguments for God are offered and presented as deductively valid and such validity does not admit of degrees.
    – Geoffrey Thomas
    Apr 14 '18 at 23:45

While none of Descartes' arguments for the existence of God is valid, I should say that the 3rd Meditation cosmological argument contains less falsity than the ontological argument.

The cosmological argument relies on causal considerations. It infers the existence of God from Descartes' having the idea of God and the assumption that only a perfect being could cause an imperfect being to have the idea of a perfect being. Deeply dubious as this argument, which I have only briefly summarised, is, it is mainstream in the sense of relying on causal premises. Most of us use causal arguments; Descartes does the same, if in this case with egregious errors of reasoning.

The ontological argument in the 5th Meditation - not called such by Descartes - is seriously different in its attempt to prove the existence of God a priori from concepts alone. This takes us along paths unknown to ordinary methods and rules of argument. A causal argument for the existence of God is one thing; an argument for the existence of God based on the idea of God's being the idea of a perfect being; on existence being a perfection; and on God's necessarily existing because a perfect being must have all perfections, including that of existence, is baffling in its leap from the idea of a perfect being to the actual existence of a perfect being on the assumption that existence is included in the very idea of a perfect being. This looks packed with falsity.

I have set out the ontological argument without the full detail in which Descartes encases it. But it is, if I may put it so, inherently less probable and more counter-intuitive than the causal argument.

Both arguments fail but if either had a chance of success, the casual argument would be the more likely, though neither argument has a probability far removed from zero.


Descartes, Meditations, ed. J. Cottingham, Cambridge.

J. Cottingham, Descartes, Oxford : Blackwell, 1986, ch. 3.

H.M. Bracken, Descartes, Oneworld Publications, 2010.

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